1. Field of the Invention
The present invention relates to a method of manufacturing an aluminum alloy sheet, more particularly, to a method of manufacturing an aluminum alloy sheet having excellent formability and excellent bake hardenability, having natural aging retardation property exhibiting no change in strength with time prior to being subjected to press forming, and suitable for use in an automobile body sheet.
2. Description of the Related Art
A conventional surface-treated cold-rolled steel sheet has frequently been used as a sheet material for an automobile body panel. In recent years, however, for the purpose of reducing fuel consumption, a light-weight automobile body panel material has been demanded. To satisfy the demand, an aluminum alloy sheet has begun being used for the automobile body panel.
Nowadays, manufacturers in-press forming of panel sheets are requesting that the material not only have low yield strength until being subjected to press forming so as to provide a satisfactory shape-retaining property [Jidosha Gijyutu (Automobile Technology), Vol. 45, No. 6 (1991), 45)], but also have a property such that strength thereof can improved during paint baking to provide satisfactory formability of deep drawing and overhang, and dent resistance.
Under these circumstances, an attempt has been made in which the strength of the material was improved by adding Cu and Zn to a non-heat treated type, Al--Mg based alloy which has superior formability to other aluminum alloys. As a result, an Al--Mg--Cu system alloy (Jpn. Pat. Appln. KOKAI Publication Nos. 57-120648, 1-225738), an Al--Mg--Cu--Zn system alloy (Jpn. Pat. Appln. KOKAI Publication No. 53-103914), and the like have been developed. These alloy sheets are superior to an Al--Mg--Si system alloy sheet but inferior to a conventional surface-treated cold-rolled steel sheet in formability, and exhibit a poor shape-retaining property since the alloy sheets have high strength prior to being press formed. In addition, the degree of hardening obtained by paint baking is not sufficient, and the degree of hardening is low only to prevent a work hardening value obtained by press-forming from lowering. In Jpn. Pat. Appln. KOKAI Publication No. 57-120648, an attempt has been made to improve the strength at the time of the paint baking by precipitating an Al--Cu--Mg system compound; however, the results have not been satisfactory. Since the effect of Si in improving baking hardness was not yet discovered at the time the aforementioned application was made, Si was limited to a low level.
A conventional 5052 material is used in the automobile body panel. Although it exhibits a superior shape-retaining property owning to low yield strength prior to being subjected to press forming, 5052-0 is inferior in dent resistance since satisfactory hardness cannot be provided by paint baking.
The above mentioned Al--Mg--Cu or Al--Mg--Cu--Zn system alloys have a common disadvantage in that the alloys exhibit a secular change in the strength prior to being subjected to press forming since natural aging starts right after the final heat treatment ["Report of 31th light metal annual symposium", Sumi-kei Giho (Sumitomo Light metal technology report), vol. 32, No. 1 (1991), 20, page 31)]. Therefore, it is necessary to control timing of the manufacturing raw material and heat treatment, and a period of time from the heat treatment to press forming.
One technique of suppressing the change in the strength with time by natural aging is provided by Jpn. Pat. Appln. KOKAI Publication No. 2-47234, which discloses that natural aging of the Al--Mg--Cu--Zn system alloy is suppressed by reducing a content of Zn, which has a significant effect on natural aging.
Nevertheless, heretofore, the Al--Mg--Cu alloy and Al--Mg--Cu--Zn alloy do not satisfy at least one of bake hardenability shape-retaining property, and natural aging retardation property, even though they may have excellent formability relatively close to that of steel.
On the other hand, another technique of improving the natural aging property as well the as bake hardenability by performing a heat treatment of two steps to an Al--Mg--Si alloy in which bake hardenability is increased by forming a GP zone of Mg2Si is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-70907.
Although the Al--Mg--Si alloy has excellent bake hardenability, the Al--Mg--Si alloy has poor formability, it is necessary to improve the formability in the alloy of the publication.